The New Statesman has an article that asked a lot of atheist luminaries and some lesser glowworms like yours truly to explain why they don't believe in gods. I don't think it's available online (I have a copy, though, and posted it outside my office door, so stop on by if you want to read it), but there is a discussion on the New Statesman blog. There are a whole bunch of entertaining short entries in the full article, but I'll just post mine — I gave them two reasons that I don't believe in gods.
1. The process. I am accustomed to the idea that truth claims ought to be justified with some reasonable evidence: if one is going to claim that, for instance, a Jewish carpenter was the son of a god, or that there is a place called heaven where some ineffable magical part of you goes when you die, then there ought to be some credible reason to believe that. And that reason ought to be more substantial than that it says so in a big book...after all, there are seven books claiming that Harry Potter is a wizard, and there aren't very many people who see that as anything but fiction. Religious claims all seem to short-circuit the rational process of evidence-gathering and testing, and the sad thing is that many people don't see a problem with that, and even consider it a virtue. It's why I don't just reject religion, but actively oppose it in all of its forms — because it is fundamentally a poison for the mind that undermines our critical faculties.
2. The absurdity. Religious beliefs are lazy jokes with bad punchlines. Why do you have to chop off the skin at the end of your penis? Because god says so. Why should you abstain from pork, or shrimp, or mixing meat and dairy, or your science classes? Because they might taint your relationship with your god. Why do you have to revere a bit of dry biscuit? Because it magically turns into a god when a priest mutters over it. Why do I have to be good? Because if you aren't, a god will set you on fire for all eternity. These are ridiculous propositions. The whole business of religious is clownshoes freakin' moonshine, hallowed by nothing but unthinking tradition, fear and superstitious behavior, and an establishment of con artists who have dedicated their lives to propping up a sense of self-importance by claiming to talk to an invisible big kahuna. It's not just fact-free, it's all nonsense.